Magnetic field detector



Aug. 18, 1959 c. L. MENGEL ET AL MAGNETIC FIELD DETECTOR V2,900,595 Patented Aug. 18, 1959 MAGNETIC FIELD DETECTGR Clare L. Mengel, Sellersville, and Dickerson P. Miles, Oreland, Pa.

Application August 31, 1955, Serial No. 531,854

7 Claims. (Cl. 324-6) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a method and apparatus for detecting anomalies in the earths magnetic field and more particularly to a method and apparatus for indicating the presence of a submarine or other target capable of creating an anomaly in the earths magnetic field.

Present techniques used in searching large areas of the earth for changes in the magnetic field generally consist of moving the detection instruments from point to point in the magnetic field. The disadvantages of this method when fused to search an area are that it is slow, expensive, and unsuitable for covering large expanses such as oceans.

The present invention proposes to detect the presence of anomalies in the earths magnetic field by employing airborne equipment to sweep large areas at a time permitting rapid movement from place to place. The invention employs two airplanes travelling parallel paths separated by a predetermined distance. The first of these two airplanes sends out two microwave radio beams at dierent wave lengths which are affected to different extents by the earths magnetic field, relying on the Faraday, Zeeman and Voigt effects. By measuring the difference in phase shifts between the two microwave frequency signals which are received by the second airplane, it is possible to evaluate changes in the magnetic field indicating the presence of a submarine or other target capable of creating an anomaly in the earths magnetic field.

An object of the present invention is the provision of a method and arrangement for remotely scanning wide geographical areas for the purpose of detecting anomalies in the earths magnetic field.

A further object of the invention is the provision of a method and apparatus for using aircraft in the detection of the presence of a submarine or other target capable of creating an anomaly in the earths magnetic field.

Still another object is to provide a method and apparatus for utilizing a pair of transmitted radio waves through an area for detecting anomalies in the earths magnetic field.

A final object of the present invention is the provision of radio equipment for transmitting and receiving two.

signals at frequencies which respond differently to changes in the earths magnetic field and comparing said two signals for differences in phase caused by anomalies in the earths magnetic field in the area swept by said signals.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawing in which is shown schematically theV arrangement for carrying out the method of this invention.

Ther present invention employs two beams of RF energy netic field than the reference wave.

at separate microwave frequencies. The speed of the two beams depends, as is well understood, on the index of refraction of lthe intervening medium. The index of refraction for the medium may vary over a range of frequencies. If one of the frequencies chosen is affected in its speed of travel by the earths magnetic field to a greater extent than the other frequency is affected, then it is possible to measure the magnetic field between the point of transmission and the point of reception of the signals by comparing the phases of the signals at the point of reception. In the present method and arrangement, one frequency is chosen at 30,000 megacycles which is defined as the reference wave and the second frequency chosen is the second harmonic of the 30,000 megacycle signal, that is, 60,000 megacycles. The latter frequency is affected more sharply by the earths mag In the region of 60,000 megacycles there is an oxygen molecule absorp tion band, and since oxygen makes up approximately 20 percent of the earths atmosphere, considerable energy is absorbed from the radiated beam in this band. This results in an accentuation of the magnetic fields effect on the refractive index of the medium.

In applying the above principles, it should be readily apparent that when an area is scanned by the reference and harmonic frequency waves, a phase difference between the two signals over a given distance will occur which can be defined as normal. By continuing the sweep, a distinct shift in this phase difference will indicate, according to this invention, that an anomaly in the earths magnetic field exists somewhere in area swept. The equipment can be adjusted for a zero reading at the expected phase difference for the considered normal field of the earth between two points. Definite signals indicating shifts in phase thereby detect the presence of a target or other object capable of producing an anomaly in the earths magnetic field. This method, of course, does not pinpoint the exact location of the target nor does it point out the exact nature of the cause of the anomaly. However, further investigation should provide the information sought.

ReferringV to the drawing showing schematically an arrangement which is an illustrative embodiment of the specific apparatus that will perform the method of the instant invention there is shown a pair of aircraft consisting of a transmitting aircraft and a receiving aircraft flying parallel to each other at some predetermined distance apart, such as 10 miles. The transmitting aircraft may hold transmitting apparatus 9 capable of transmitting a signal of given frequency and a second harmonic of that signal. For example, the transmitting aircraft, as indicated in the drawing, may emit a first signal at 30,000 megacycles and a second signal at 60,000 megacycles, these frequencies having been found most desirable for the purpose of the invention. Other frequencies, having desirable properties, may of course be chosen instead. The 30,000 and 60,000 megacycle signals are received by the receiving aircraft in separate receivers, 10 and 11, respectively. According to the principles of this invention, the 30,000 megacycle signal is only slightly affected in phase by the earths magnetic field while the 60,000 megacycle signal is appreciably shifted in phase, for reasons explained above. A local oscillator i2 may supply a signal intermediate the original two signals, as for example, a signal at 45,000 megacycles. This interme diate signal may be beat with the two incoming signals received respectively at receivers i0 and 1l producing a pair of intermediate frequencies at 15,000 megacycles 3 v 180 in phase from its original phase. The other intermediate frequency signal may be fed through a phase restorer to take care of normal shifts of phase due to the earths magnetic held and other distortionefects A phase comparison network 17V compares the two in'- 5 terinediate frequency signals which cancel each other out when exactly 180 out of phase; A reading on voltmeter 18 ndicatesa phase'difference other 180. In op eration of this arrangement the transmitting aircraft and the receiving aircraftmay be own at a distance of'lO 10 miles apart and phase restorer 1S adjusted to 'present a zero voltage output at 18 in the presence of only'the earths magnetic field at a particular locality;

In the presence of a submarine or other targetcapaf ble of creating an anomaly in the earths'm'agnetic field, V 15 the 30,000 megacycle signal and 60,000Amegaeycle'sigl nal will be received byV receivers 10 and 11, Vas alre'adyV explained. Howeven the 60,000 'megacycle signal 'will be shifted in phase to a'farV greater extent than the @,000

megacycle signal in the presenceof suchan l'anomalyfzo for the Vreasons already explained. Therefore', at the phase comparison network 17 the 60,000 megacycle signal which has been reduced to an intermediate `frequency signal at 15,000 megacycles willshow Va greater shiftl ing from the origin than the 30,000 megacycle signal and,

therefore, a voltage diierent from Vzero will occur at the output of the phase'comparison network. This voltage will serve to move the indicator of the voltmeter 18, thereby indicating presence of an anomaly in the magnetic vfield between the vtwo aircraft and the probable presence of a submarine in that'area.

It is understood that the present invention is a highly efficient manner of sweeping large areas quickly and ef-Y ciently for locating the probable presence of submarines.

This method as presently envisioned would not necessarily supplant the'present submarine detection equip-V ment for pinpointing a submarine for destruction. How'- i ever, the present invention by virtue of its simplicity, efJ ciency and ability to sweep large areas to detect the nal from a iirst point in the earths magnetic field, means Vto radiate a second signal at a different microwave frequency from said first point, means to receive said rst and second signals at a second point remotely removed from said first point, means to convert said signals into rst and second intermediate frequency signals of identical frequency, meansto shift one ofi-said intermediate frequency signals 180 in phase, means to compare phases of said intermediatefrequency signals, means to adjust the phase of the other of VVsaid intermediate frequency signals causing. zero phase difference ,inA saidfintermediate frequency signals when said earths magnetic field is con- .sidered normal, and means to 4indicateffurther unequal phase shifting .in said signals earths magnetic field.

2. Apparatus for indicating anomalies in the earths magnetic eld comprising a first point and a second point caused by anomalies in the in the earths magnetic iield separated by a predetermined distance, a`rst'mic'rowave transmitting means locatedat` said iirst point for radiating a first microwave frequency signal in a band which is only slightly aifecte'd by`changes in said magnetic eld, a second Vmicrowave transmitting Vmeans located at said iirst point for radiating a second second signal and reducing said second signal to afurth signal at said given intermediate frequency, means Ycornparmg the phases of said intermediate frequency signals, means recording the difference in said phases, and means for moving one of said two points for sweeping an area lof the earth with said transmitted Asignals causing a readprobable presence of a submarine or other target such as 40 surface vehicles in time of fog, permits a more ecient screening of convoys and prevents submarines from coming close enough to' wreak havoc on surface craft.

Obviously, other means for detecting the difference in effect of anomalies in the earths magnetic iield are possible without deviating from the instant invention. The

arrangement shown is merely an illustrative embodiment of the arrangement for detecting the preferential phase shift occurring on the higher frequency signal. Alternate schemes might include the modulation of one RP" carrier by another lower frequency carrier and creating two side bands, one which falls in an absorption and an other outside the absorption band, the measurement be-F ing by similar techniques. Also, other Vehicles or sta# tions in addition to aircraft may be used to carry the equipment.

Thus, it is apparent that herein is proposed a new and a more eiiicient method and apparatus for searching the earths magnetic eld for anomalies or changes in the earths magnetic field by radio frequency propagation and the corresponding effect that the earths magnetieeld and changes thereof has on radio frequency propagation. Of course, withV proper calibration of equipment, the ap -paratus and method of this inventionmay beusedto integrate the magnitude of the earths magnetic eld between the sending and receiving points. Y Obviously, many modifications and Variations of the present invention are possible in the light of the above,y

teachings. It is, therefore, to'be understood that the scope of the appended claims the'inventionmayV be 70 practiced otherwise than as specifically described.

What is claimed is:

1. ApparatusV for detecting Yanomalies earthis i magnetic iield indicating presence of a submarine target comprising means to radiate a microwave frequency sig-V ing on said recording means indicative of the magnitude of said earths magnetic field in said area. f

3. Apparatus for indicating anomalies in the earths magnetic iield comprising a first point and a second point 1n the earths magnetic lield separated byY a predeter mined distance, a iirst microwave transmitting means located at said rst point for radiating a lirst microwave frequency signal Yin a band which is only slightlyfaiected by changes in said magnetic lield, a second Yrnicrciwavt. i Ytransmitting means located at said irst point for radiating a second microwave frequency signal simultaneously with said rst signal and in an absorption band highly sens1t1ve to VVchanges in'said magnetic eld causingdeviation of the refractive'index ofthe medium in said bandfa iirst mlcrowave receiving means located "at'said second point for receiving said lirst signal and 'reducing saidjirst signal to 'a' third signal at a'given predetermined intermediate frequency, a second microwave reeivinggmeans located at said second pointfor "receiving'safid sedond signal and reducing said second signal 'to a fourth (signal at said given intermediate frequency, niean's comparing the phases of said intermediate frequency signals, means recording the difference in said phases, means. correctingVV the phase of one of said intermediate frequencvsignals to match the phase ofthe votheof said intermediate fre-I .quency signal's'causinga alerofreadingon aidV readingmeans for indicating a chosen normal 4intensity Ofmag-yV netic iield between said two points, andmeansfor moya Y,

Ving one of said 'tw points'for sweeping anarea ofthe earth with s ai'd transmitted signals causing ,areading other than zero Yonfsairl recording meanswhen Ian anomalyin said earthsmagnetieelgl.separating said. two points iS SWGRL. 4. vAnnethod. of detecting submarinesfcomprising Vthe-v Y steps of simultaneously radiating two signalsY at two dif-j ferent VI nicrowaye frequencies, one of the frequencies being a second harmonic of the other frequency, receiving said signals at a point removed from said source of signals, comparing the phase shift of each of said signals, and producing a signal proportional to the relative phase shift of said two signals for indicating changes in the earths magnetic field swept by said radiated signals,

5. A method for integrating the magnitude of the earths magnetic field between a first and second point in Said field comprising the steps of radiating from said first point a first microwave frequency signal simultaneously with a second microwave frequency signal in a portion of the microwave frequency band more sensitive to changes in intensity of said magnetic field than said first signal and said second signal being a second harmonic of said rst signal, receiving said rst and second signals at said second point, and comparing the phases of said received signals for indicatingrrelative phasenshifting in both of said signals due to the intensity of the magnetic field separating said points.

6. A method for integrating the magnitude of the earths magnetic field between a rst and second point in said field comprising the steps of radiating from said first point a first microwave frequency signal simultaneously with a second microwave frequency signal in a portion of the microwave frequency band more sensitive to changes in intensity of said magnetic field than said first signal and said second signal being a second harmonic of said first signal, receiving said first and second signals at said second point, adjusting the phase of one of said signals to be in fixed relation relative to the phase of the other of said signals when the area between the two said points has an earths magnetic field which is considered normal, moving at least one of said points to sweep an area with said signals, and comparing the phases of said received signals for indicating relative phase shifting in said signals due to changes in the magnitude of the mangetic field swept by said signals.

'7. Apparatus for detecting anomalies in the earths magnetic field indicating presence of a submarine target comprising means to radiate a first microwave frequency signal from a first point in the earths magnetic eld, means to radiate from said first point a second microwave signal being a harmonic of said first signal, means to receive said first and second signals at a second point, means to compare phase shift of said first signal with that of said second signal caused by the earths magnetic field, and means to indicate departure from normal phase shift difference, due to anomalies in said earths magnetic field.

References Cited in the le of this patent UNITED STATES PATENTS 1,816,579 Hammond July 28, 1931 2,426,918 Barret Sept. 2, 1947 2,526,425 Schultheis Oct. 17, 1950 2,582,350 OBrien Ian. 15, 1952 2,623,924 Cartier Dec. 30, 1952 2,642,477 Puranan June 16, 1953 2,794,949 Hedstrom June 4, 1957 FOREIGN PATENTS 507,468 Canada Nov. 23, 1954 E (SEAL) KARL- 14. AXLINE UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Paten-'t No. 2,900,595 August, le, 1959 Clare lL. Mengel ei al.l

lt is hereby certified ahat error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Pa'en; Should read as corrected below. Column 4, line 24, claim 2, strike .out "in phase"; line 64, claim 3, for ree5ldin;",v second occurrence, read recording column line 5, claim for mangetic" read magnetic Sig-ned and sealed this 12th day of July 1960.

ROBERT C WATSON Conmissioner of Patents Attesting-.Offcer UNITEDv STATESv PATENT oRFTcE CERTTFICATEOF CORRECTION Paten-'u No. 2,900,595 T August 15, 1959 Column 4, line 24, vclaim 2, strike out "in phase"; line 64, claim 3, for reading"f," second occurrence, read recordlng --3 column 6, line 5, claim 6, for "mangetc" read magnetic f signed and sealed This 12th day of July 1960.

l (SEAL) KARLTH. AXLINE ROBERT C WATSON Attes'tingjjofficer lConrm`.s,soner of Patents 

